1. Field of the Invention
The invention is concerned with hard magnetic materials, processes for shaping such materials, and devices utilizing materials so shaped. Shaping is accomplished by steps including working with at least some critical part of the working being conducted at low temperature, sometimes at room temperature. Magnetic properties are sufficient to permit use in many magnetically biased devices, such as, electroacoustic transducers, including receivers, loudspeakers, and the like.
2. Description of the Prior Art
The development history of hard magnetic materials is characterized as a continuing search for higher and higher values of coercivity, remanent magnetization, and energy product. This is evident from consideration of such devices as permanent magnet loudspeakers, where increased energy product results in improved bass response for given magnet size. In receivers too, engineering design considerations, such as air gap and volume, suggest increasing values of coercivity, as well as of energy product. The quest has been accelerated by recent design trends, all of which lead toward increasing miniaturization which, in turn, indicate larger energy product, as well as coercivity to accomplish a desired permanent bias for now reduced proportions.
From many standpoints, it is reasonable to characterize modern, permanent magnet materials as having coercivities of the order of at least 250 Oe. and remanent magnetizations of at least 7,000 Gauss, indicating a maximum energy product of at least 1 million Gauss-oersteds. For real operating devices, the energy product value of concern is that measured along an operating line (or load line) which depends upon design parameters, such as, circuit reluctance, etc.; and here a useful energy product may be somewhat less than the maximum value.
From the standpoint of processing, hard magnetic materials may be classified as belonging to either of two categories. Brittle alloys, are exemplified by the Alnico series (see R. M. Bozorth; Ferromagnetism, D. Van Nostrand, 1951). Such compositions, based on aluminum, nickel, and cobalt do not lend themselves to working, e.g., by rolling, or drawing. Thus piece parts of such alloys are most expeditiously or necessarily formed by casting or powder metallurgy. Ductile alloys, exemplified by the alloys: Cunife (cobalt, nickel, copper and iron), Cunico (cobalt, nickel, and copper) and Vicalloy (vandium, cobalt and iron), can be worked readily at room temperature. Piece parts of such alloys are generally processed by operations such as flat rolling and wire drawing.
From a commercial standpoint, other fabrication approaches are sometimes indicated. An example involves Remalloy, an alloy of iron, cobalt, and molybdenum--e.g., 20 weight percent molybdenum, 12 weight percent cobalt, and the remainder (to equal 100 weight percent) iron. Piece parts of Remalloy, which is in the brittle category, are produced by working which, however, requires temperatures exceeding 1,100.degree. C. This exemplary Remalloy composition, already reflecting a compromise between workability and maximization of magnetic characteristics, is notably used in telephone receivers. This alloy is typically formed into a rolled hot band of the order of 100 mils in thickness by a series of steps that include (1) casting of ingot; (2) hot rolling at 1200.degree. C to the desired thickness in a series of rolling operations; (3) stamping to desired configuration with the stamping operation necessarily carried out at elevated temperature; (4) solution heat treatment at 1200.degree. C; (5) grinding to final dimensions; and (6) finally, a terminal heat treatment near 700.degree. C to develop the permanent magnetic characteristic. Such a Remalloy piece part, designed, for example, in the telephone receiver, may have a coercivity of 300 Oe., a remanence of 9,000 Gauss, and a usable energy product of perhaps 1 million Gauss-oersteds.
Hot workable Remalloys, processable as described, are characterized by magnetic properties among the best obtainable for hot workable materials, at least for materials within an acceptable price range for mass production. For certain uses where piece parts are subject to shock, even hot workable Remalloys are unacceptable; and so, for example, even the handset receiver used as an example above, may not be constructed of Remalloy for certain uses, for example, for use in pay telephones where abuse may be expected.